(1) Field of the Invention
The present invention relates to a light-colored water based intercoat coating composition showing excellent properties in a low volatile organic compound content and in coating composition stability, and capable of forming a coating film showing excellent properties in anti-chipping properties, yellowing resistance and beautiful appearance, and to a multi-layer coating film formed by use of the light-colored water based intercoat coating composition.
(2) Description of Background Art
The automobile body is usually coated with a multi-layer coating film comprising an undercoat coating film with an electrodeposition coating composition, an intercoat coating film and a topcoat coating film for the purpose of imparting anti-corrosive properties and beautiful appearance. While an automobile is running, pebbles, gravels, anti-freezing agents, ice bumps, etc. on the road may rebound so as to collide against the automobile body, resulting in drawbacks of making mars on the multi-layer coating film, locally braking and peeling off the multi-layer coating film, showing poor appearance, exposing a metal substrate, developing rusts for corrosion (these drawbacks may be called as “chipping” hereinafter).
On the other hand, a coating composition using an organic solvent has mainly been used in the art. Since the use of the organic solvent may have a matter of concern about influences on health and environment due to a solvent vapor volatilized into the atmosphere, the use of a water based coating composition for use in the automobile has been advanced, and in the field of the intercoat coating composition for use in the automobile, developments of a water based intercoat coating composition imparted with various kinds of functions have been made.
Japanese Patent Application Laid-Open No. 209066/96 corresponding to U.S. Pat. No. 5,705,595 discloses a water based intercoat coating composition by use of a blocked isocyanate such as an aliphatic polyisocyanate oligomer, adduct of the oligomer, adduct of aliphatic polyisocyanate and the like, for the purpose of improving anti-chipping properties.
U.S. Pat. No. 5,455,297 discloses a water based coating composition using an aqueous isocyanate.
Some water based intercoat coating compositions use a mixture of water with a water-soluble organic solvent as a solvent from the standpoints of coating composition stability and coating workability, wherein an amount of the water-soluble organic solvent is desired to be reduced to a minimum.
Recently, a prior art reference (see Coating and Coating Composition, October, 2000, pages 16-27, Trend of Water Based Polyurethane Resin) reports on reduction of an amount of an organic solvent in the coating composition or reduction of volatile organic compound (VOC), as well as anti-chipping properties.
Reduction of the volatile organic compound or a volatile solvent in the water based intercoat coating composition is strongly demanded, and developments of a water based intercoat coating composition capable of providing improved anti-chipping properties are demanded. However, simple combinations of the above prior art would be impossible to develop a coating composition well balanced in anti-chipping properties, low volatile organic compound content, water resistance and coating composition stability.
Recently, in a multi-layer coating film for use in an automobile outer plate, a white pearl coating color using a mica as a luster agent is becoming a mainstream, and for the purpose of imparting anti-chipping properties, a blocked polyisocyanate curing agent is used in the intercoat coating composition, wherein a blocking agent such as methyl ethyl ketoxime, alcohol and the like is used. The use of the methyl ethyl ketoxime as the blocking agent may produce a problem of a film yellowing due to heat curing, resulting in making difficult a white pearl coating color design in a coating composition having anti-chipping properties. The use of alcohol as the blocking agent may not result yellowing, but a high dissociation temperature of the blocking agent makes it impossible to obtain a satisfactory curing properties in a temperature range of curing the automobile intercoat coating composition.
A Japanese Patent Application Laid-Open No. 512772/99 corresponding to WO97/12924 discloses a white paint composition containing an acrylic emulsion, a blocked isocyanate blocked with a 3,5-dimethylpyrazole blocking agent and the like, and a coating method which comprises coating the composition onto a coating substrate, followed by curing at 120° C. for 45 minutes, but discloses neither the use of a specified polyester resin containing as essential components an alicyclic polybasic acid, an alicyclic polyhydric alcohol and the like, nor improvements in anti-chipping properties by the use of the polyester resin.
Japanese Patent Application Laid-Open No. 2000-26570 corresponding to EP 0942023 discloses that a transparent coating composition containing a hydroxypolyester, a 3,5-dimethylpyrazole-blocked polyisocyanate crosslinking agent and the like is coated onto a coating substrate, followed by heat curing at 120° C. for 30 minutes to form a coating film showing higher film hardness, improved solvent resistance and higher corrosion resistance compared with a transparent coating composition containing a hydroxypolyester, butanoneoxime-blocked polyisocyanate crosslinking agent and the like, but discloses neither the use of a specified polyester resin containing as essential components an alicyclic polybasic acid, an alicyclic polyhydric alcohol and the like, nor improvements in anti-chipping properties by use of the polyester resin.
Japanese Patent Application Laid-Open No. 2001-181568 corresponding to EP 1110987 discloses a coating composition containing a condensation product of a carboxyl group-containing such as respectively carboxyl group-containing polyester resin, polyurethane resin, acrylate resin and the like with a hydroxyl group-containing resin such as respectively hydroxyl group-containing polyester resin, acrylic resin, polyurethane resin, epoxy resin and the like, and a mixture of a water-insoluble blocked isocyanate curing agent and a hydrophilic blocked isocyanate curing agent respectively blocked with a known blocking agent, discloses a method which comprises coating the coating composition onto a metal substrate to form a surfacer coating film, discloses that the use of the 3,5-dimethylpyrazole blocking agent particularly preferable because of showing non-toxicity and no yellowing at 180° C. or higher, and discloses that the combination of the water-insoluble component with the hydrophilic component results improvements in gloss and anti-stone chipping properties of the resulting multi-layer coating film, but discloses neither the use of a specified polyester resin containing as essential components an alicyclic polybasic acid, an alicyclic polyhydric alcohol and the like, nor improvements in anti-chipping properties by use of the polyester resin.
Japanese Patent Application Laid-Open No. 304843/95 discloses, as an example, that a transparent coating composition prepared by reacting a mixture of a hydroxyl group-containing polyacrylate resin and a polyester polyol with a blocked polyisocyanate crosslinking agent obtained by blocking 1,6-diisocyanatohexane with a blocking agent mixture of 1,2,4-triazole, 3,5-dimethylpyrazole and methoxypropyl acetate at an equivalent ratio of NCO/OH was subjected to a thermal yellowing measurement after a curing procedure and a thermal yellowing measurement after an overbaking procedure with the result that the above thermal yellowing is remarkably reduced compared with the case where a butanoneoxime is used as a blocking agent, discloses that the blocked polyisocyanate crosslinking agent obtained by blocking with the blocking agent mixture is preferable for use in a topcoat in an automobile multi-layer coating film, but does not disclose the use of the above blocked polyisocyanate crosslinking agent in an intercoat, and discloses neither the use of a specified polyester resin containing as essential components an alicyclic polybasic acid, an alicyclic polyhydric alcohol and the like, nor improvements in anti-chipping properties by use of the polyester resin.
Japanese Patent Application Laid-Open No. 225509/96 discloses a method of preparing a polyisocyanate at least partly blocked with 3,5-dimethylpyrazole, exemplifies other blocking agents such as butanoneoxime, 1,2,4-triazole, diisopropylamine, diethyl maleate, ethyl acetoacetate and the like, discloses that a laquer polyisocyanate blocked with 3,5-dimethylpyrazole is usable as a crosslinking agent for one component polyurethane coating composition, and makes possible a considerably low thermal yellowing of a coating and a comparatively low curing temperature of about 130° C. compared with other blocking agents such as butanoneoxime and the like, but does not disclose the use of the blocked polyisocyanate obtained by blocking with 3,5-dimethylpyrazole as a crosslinking agent for an intercoat coating composition, and discloses neither a combination thereof with a specified polyester resin containing as essential components an alicyclic polybasic acid, an alicyclic polyhydric alcohol and the like, nor resulting improvements in anti-chipping properties.
Japanese Patent No. 3292886 discloses a water based intercoat coating composition containing as essential components a hydroxyl group-acid group-containing polyester resin, amino resin, alicyclic epoxy compound, neutralizing agent, pigment and water, and discloses that the addition of the alicyclic epoxy compound to the coating composition known in the art makes possible for the epoxy group in the epoxy compound to trap a basic substance and to take place a usual reaction between amino group and hydroxyl group, resulting in making it possible to show good performances in coating film curing properties, satisfactory anti-chipping properties as an intercoat coating film and the like, but discloses neither the use of a specified polyester resin containing as essential components an alicyclic polybasic acid, an alicyclic polyhydric alcohol and the like, nor resulting improvements in anti-chipping properties, and discloses neither the use of the blocked polyisocyanate curing agent, particularly the blocked polyisocyanate curing agent obtained by blocking with the 3,5-dimethylpyrazole blocking agent, nor resulting improvements in yellowing resistance.